This was solved in the ROS 6.41rc37
What’s new in 6.41rc37 (2017-Oct-02 06:47):
*) fetch - accept all HTTP 2xx status codes;
Now we may run fetch each after another, if web server return 202 and other 2xx status codes:
/tool fetch url="http://httpstat.us/202" keep-result=no
/tool fetch url="http://httpstat.us/202" keep-result=no
/tool fetch url="http://httpstat.us/202" keep-result=no
Thank you Mikrotik team!